An audio amplifier (also referred to as a power amplifier) is used to amplify an audio signal to drive an electroacoustic transducer such as a speaker or a headphone. FIG. 1 is a circuit diagram of an audio output device 200R including an audio amplifier 100R of a BTL (Bridged Tied Load) type. The positive electrode terminal (+) and the negative electrode terminal (−) of an electroacoustic transducer 202, which is a load, are connected to the output terminals OUTP and OUTN of the audio amplifier 100R, respectively. The audio amplifier 100R includes a first amplifier 102P that outputs a drive voltage VOUTP, and a second amplifier 102N that outputs a drive voltage VOUTN having a phase reverse to the phase of the drive voltage VOUTP.
The audio amplifier 100R further includes a load connection state detection circuit 106 for detecting abnormality such as an open state of the electroacoustic transducer 202. For example, a conventional load connection state detection circuit may cause a current to flow into one output terminal to detect one of a normal state, an open state, and a short state in accordance with a voltage or current appearing at the one output terminal.
The present inventors have reviewed the audio amplifier 100R of FIG. 1 and recognized the following problems.
In the application circuit of FIG. 1, the two output terminals OUTP and OUTN are directly connected to the terminals + and − of the electroacoustic transducer 202, respectively. However, in some different application circuits, a DC blocking capacitor may be interposed between the output terminal OUTP and the terminal + of the electroacoustic transducer 202 or between the output terminal OUTN and the terminal − of the electroacoustic transducer 202. In the related art, in such cases, the connection state of a load cannot be determined correctly.
In addition, in the related art, open/short criteria are defined as parameters inside the audio amplifier 100R and cannot be set by a designer of an audio output device such as the audio output device 200R.